I only ever came across one stale listing in a Belgian classified page that involved original helmet decals. The closest I have been able to find otherwise were some transfers sold by a modelling shop in the UK. One of the sizes was very close to one of the formats found on Belgian helmets.

I agree, the sticker types should be fairly easy to replicate. The water transfers, which I assume are much earlier, probably not so much. There is quite a wide variety of different styles and dimensions overall. And that doesn't even count the painted-on flags.